I. Field of the Invention
This invention relates generally to an adaptive control system for improving the performance of an internal combustion engine, and more specifically to a closed-loop control system in which both spark timing and air/fuel ratio or exhaust gas recirculation are simultaneously adjusted to achieve the enhanced performance.
II. Discussion of the Prior Art
In the Schweitzer et al. U.S. Pat. No. 4,026,251, a closed-loop digital electronic control system for an internal combustion engine is described. In that system, a machine-controlling parameter is perturbated (dithered) about a given setting and the performance of the machine is monitored to determine whether movement of the machine controlling parameter about the given setting results in an improved or degraded performance. When a given movement of the machine control parameter setting results in improved performance, the resulting control signal developed by the electronic system is used to create a further movement of the control setting in the same direction. However, if the small change introduced results in degraded performance, then the machine setting is moved in the opposite direction.
An improved version of the system of the Schweitzer et al. patent is described in U.S. Pat. No. 4,575,800 to David B. Kittelson. Instead of relying upon a fixed dither frequency, a variable frequency dither cycle synchronized with the engine's normal operating cycle is selected. By providing a shaft rotation sensor, each dither cycle begins with respect to a predetermined shaft angle and each phase of the dither cycle (advance and retard) is comprised of an equal number of firing pulses. Using this approach, the engine's cylinder firings do not continuously move in and out of phase with respect to the dithering of the machine control parameter which serves to reduce the error signal, especially as compared to the system of the aforereferenced Schweitzer et al patent.
In the Kittelson system, the ignition timing is automatically adjusted to provide minimum fuel consumption for a range of operating conditions, fuels, and air/fuel ratios. It works by advancing and retarding the spark timing (dithering) and monitoring the engine response to the timing changes. If the system determines that the engine speeds up during the advance phase of the dither, a small advance correction is applied to the base timing. If the engine slows down during the advanced phase of the cycle, a retard correction is made. These corrections continue until no speed changes are detected.
Those skilled in the art appreciate that internal combustion engines require air/fuel ratio, ignition timing and (in some cases) exhaust gas recirculation rate (EGR) be adjusted properly to achieve optimum (minimum) fuel consumption. Where the operating conditions of the engine are well defined and the fuel properties do not change, it is possible to derive "maps" of the optimum ignition timing and air/fuel ratio or EGR for a given engine. However, if operating conditions change outside of the range of the maps or ambient conditions change and are not corrected for, or if engine-to-engine variations exist due to manufacturing tolerances, wear, deposit accumulations or if the fuel properties change, mapped systems will not be able to keep the spark timing or air/fuel ratio at optimum. The Holmes U.S. Pat. No. 4,893,600 assigned to Lucas Industries plc describes an optimization scheme based upon the dithering principle, and which requires that an engine performance map relating to varying conditions be stored in a memory and utilized during the control sequence.
The air/fuel ratio presents even a greater challenge. Although the optimum air/fuel ratio can be found for a range of operating conditions, there are currently no simple feedback sensors to determine if the proper air/fuel ratio is being maintained. Moreover, changes in fuel properties and ambient conditions can substantially change the required air/fuel ratio. For these reasons, mapped systems simply cannot adjust for such changes.
As is described in the Schweitzer et al. U.S. Pat. No. 4,026,251, an adaptive control system for optimizing internal combustion engines of the type used in adjusting spark timing may also be used to control the air/fuel ratio. Thus, if the air flow to the engine is dithered while the fuel flow is held constant, and corrections applied in a fashion similar to the timing adjustment described above, the air/fuel ratio for minimum fuel consumption can be determined. If the fuel is dithered with constant air flow, then the maximum power air/fuel ratio can be reached. If the base air or fuel flows are changing due to non-steady engine operation, the optimum air/fuel ratio can still be determined by comparison of the rates of change in engine speed during the different phases of the dither cycle. If it is found that the engine speeds up more during the lean portion of the cycle than during the rich portion, correction in the lean direction is made. On the other hand, if the engine slows down during the lean portion of the dither cycle, correction is made in the rich direction. The implementation of a system for optimizing air/fuel ratio requires a method for dithering the engine air flow while maintaining constant fuel flow.
An adaptive, optimizing system for both air/fuel ratio and ignition timing is found to be particularly useful in applications in which the fuel properties are variable. Fuel properties are difficult for engine control systems to measure and can have large effects on the optimum air/fuel ratio and timing. Where the control system is unable to measure fuel properties, such as in so-called flexible fuel systems, a mapped system, such as described in the aforereferenced Holmes '600 patent, is of little use in determining the proper air/fuel ratio or timing. Applications where fuel properties are variable include natural gas fueled engines and engines fueled by waste gas, such as oil refinery byproducts, or bio-gas from landfills. In these cases, the fuel is made up of a variety of components with the concentrations of these components varying unpredictably.
Those skilled in the art appreciate that the exhaust gas recirculation rate is employed chiefly to achieve NO.sub.x control. However, the optimum EGR rate can also result in improved fuel economy. For engines designed for stoichiometric operation, the air/fuel ratio is held constant using an oxygen sensor and feedback control. In such a system, simultaneous optimization of timing and EGR can be advantageous. In fact, depending upon the design of other parts of the control system, optimization of timing, A/F ratio and EGR rate in various combinations proves attractive.